1. Field of the Invention
The present invention relates to a bandwidth control system, especially to a system including a plurality of pre-stage packet multiplexers which receive packets from user devices, and a post-stage packet multiplexer which receives packets from the plurality of pre-stage packet multiplexers and transmits the received packets to a network.
2. Description of the Related Art
In data transfer on a network, one of techniques for implementing data transfer reflecting contracts with users is bandwidth control. One example of the bandwidth control is that, when a plurality of users concurrently transmit packets in a specific bandwidth and total data amount exceeds the bandwidth, the system ensures a contracted minimum bandwidth for each user in accordance with the respective contract made in advance, without applying a first-come-first-served principle. Another example is that, even when there is no packet transmission by other users, the system does not allow a single user to monopolize a specific bandwidth and sets an upper limit to the bandwidth for each user.
In recent years, with the increase in the number of users using networks, a packet multiplexer conducting the above-described bandwidth control is configured with not a single packet multiplexer but a plurality of packet multiplexers as well as a multi-stage configuration to meet such a large number of users.
Specifically, such a packet multiplexer has a dual structure including a pre-stage and a post-stage. The pre-stage is consisted of a plurality of packet multiplexers which receive packets from users and transmit the packets to the post-stage. The post-stage is consisted of a packet multiplexer which receives the packets from the pre-stage and transmits them to a network. The pre-stage packet multiplexer performs bandwidth control under contracts with users and the post-stage packet multiplexer performs a predetermined bandwidth control.
As a technique concerning bandwidth control, Japanese Unexamined Patent Application Publication No. 2002-252625 discloses a technique wherein buffers are produced as many as the number of virtual local area networks (VLANs) allocated to physical lines connected to output ports of a rooter, and packets accumulated in the buffers are transmitted by a round-robin approach.
In the above-described related art, a problem occurs in that fairness cannot be ensured among users connected to different pre-stage packet multiplexers. Another problem is that it is difficult to increase the number of users. Specifically, fairness among users connected to the same pre-stage packet multiplexer may be ensured, but in a specific situation (e.g., when the bandwidth is congested in one pre-stage packet multiplexer and is uncrowded in another pre-stage packet multiplexer), unfairness occurs among users who have made the contracts on the same condition but connected to different pre-stage packet multiplexers. Also, since installing more pre-stage packet multiplexer(s) requires a large capacity of packet buffer for the post-stage packet multiplexer, it is not easy to increase the number of users. Even if the technique disclosed in the above-described patent document is applied, these problems cannot be solved.